Multiphase pressure boosting pump

ABSTRACT

The invention provides a system for multiphase pumping, preferably for subsea application, distinctive in that it comprises a multiphase pressure boosting pump and a separator, preferably adapted for subsea operation, a multiphase fluid inlet connected to the pump, a multiphase outlet from the separator, an outlet of the pump arranged to an inlet of the separator, a liquid outlet of the separator arranged to a liquid inlet on the pump, and a power recovery turbine arranged between said liquid outlet of the separator and said liquid inlet on the pump. The invention also provides a multiphase pressure boosting pump, preferably adapted for subsea operation, comprising a motor, one or more impellers arranged on a shaft common with a motor shaft or coupled to the motor, a pressure housing, a multiphase fluid inlet and an outlet, distinctive in that the pump further comprises a liquid injection inlet and a power recovery turbine, with the power recovery turbine arranged between the liquid injection inlet and the pump impellers.

FIELD OF THE INVENTION

The present invention relates to pressure boosting of fluids. More specifically, the invention relates to pressure boosting of multiphase flows from wells and hydrocarbon production systems, particularly subsea, the fluids can have a very wide or unpredictable range of composition, from gas to liquid. The invention provides a pressure boosting pump and a system particularly feasible for subsea pressure boosting of so called wet gas, however, the pump and system can boost the pressure of fluids having composition from pure liquid to dry gas.

BACKGROUND OF THE INVENTION AND PRIOR ART

Subsea pumping and compression is already a technology that is tested and qualified. Subsea pumping is implemented and subsea compression is scheduled to be implemented in 2015. For subsea pressure boosting of well flow or subsea system flow of wet gas of composition that is variable and in a range of mixed gas and liquid, the state of the art concept prescribes either to have at least two turbomachines arranged subsea, namely a pump and a compressor, in addition to at least one separator and other equipment or a multiphase pump with recirculation of liquid. A wet gas fluid will typically have a GVF (gas volume fraction) of 70-100%, typically about 95%. It is for wet gas subsea pressure boosting possible to use helicoaxial pumps, however, a limitation with helicoaxial pumps is the pressure boosting capability at high GVF, particularly toward 100% GVF for which there is hardly any pressure boosting at all. For pressure boosting subsea in cases of high GVF, it is known to recirculate liquid in order to enable multiphase pumps to increase their operational envelope. However, this is very inefficient and expensive since the pressure of the recirculated liquid must be choked down from the outlet pressure to the inlet pressure of the multiphase pump, and it is far more energy consuming to pump the liquid than to compress the gas, resulting in a huge loss of energy. Also positive displacement type pumps can be used, such as a twin screw pump, however, lack of robustness is a limiting factor. Currently it is not possible to handle large variations in fluid composition and flow rate effectively and reliably subsea with only one turbo machine. Accordingly, subsea separation and separate pumping and compression is a requirement in order to handle large composition variation effectively and reliably, including high water contents, such as below 70% GVF. An increasing number of turbo machines and other equipment decrease the mean time between failures. Increasing complexity also decreases the mean time between failures. For operation subsea, the reliability of the equipment is typically the single aspect of greatest importance, because failure of equipment subsea can have dramatic effect with respect to production, economy and sometimes environment and safety.

NO 307226 B describes a system for multiphase pumping, comprising a multiphase pump, a diversion element that reduces dissolved gas in the liquid, a circuit for recirculation of a part of the pumped multiphase fluid, a buffer tank and a element such as a mixing-ejector for utilization of a part of the energy diverted from the liquid. However, this system does not comprise a turbine for the utilization of energy content in the circulating fluid.

NO 168965 B describes an apparatus for pumping fluid with liquid and gas phases. Further the apparatus comprises a pump and a turbine driven by the same shaft, where a gas rich fluid is mixed with a liquid rich fluid, before being pumped to a separator. However, the arrangement apparatus according to NO 168965 B results in a poor power efficiency.

A demand exists for subsea pressure boosting equipment of increased simplicity, versatility and reliability compared to state of the art equipment. The objective of the invention is to meet the demand.

SUMMARY OF THE INVENTION

The invention provides a system for multiphase pumping, preferably for subsea application, distinctive in that it comprises a multiphase pressure boosting pump and a separator, preferably adapted for subsea operation, a multiphase fluid inlet connected to the pump, a multiphase outlet from the separator, an outlet of the pump arranged to an inlet of the separator, a liquid outlet of the separator arranged to a liquid inlet on the pump, and a power recovery turbine arranged between said liquid outlet of the separator and said liquid inlet on the pump, where the pressure at the outlet of the turbine is equal to or substantially equal to the pressure at the inlet on the pump.

The invention also provides a multiphase pressure boosting pump, preferably adapted for subsea operation and particularly feasible for implementation in the system of the invention, comprising a motor, one or more impellers arranged on a shaft common with a motor shaft or coupled to the motor, a pressure housing, a multiphase fluid inlet and an outlet, distinctive in that the pump further comprises a liquid injection inlet and a power recovery turbine, with the power recovery turbine arranged between the liquid injection inlet and the pump impellers.

Additionally, the invention provides use of any feasible pump in combination with a turbine, for regaining energy from recirculated liquid or other higher pressure liquid flow, arranged and used as described or illustrated in this document.

With the preferred embodiments of the system and pump of the invention, gas is compressed by rotating liquid in the pump impellers whilst 60-90%, typically 75% of the energy of the recirculated liquid is regained. For a typical wet gas multiphase fluid, about 50% of the motor energy is replaced by energy regained by the turbine, resulting in substantial potential for savings in the power and utility requirements and supply chains.

The pump preferably comprises multiphase impellers or blades, and each impeller preferably comprises at least one blade or radial fluid conduit. The impellers are preferably according to the teaching of WO 2011/000821, to which reference is made. The power recovery turbine is preferably arranged on the pump shaft, alternatively a separate turbine shaft with a generator is coupled to a motor or a motor shaft. The power recovery turbine delivers the injected liquid, or liquid rich flow of 0-10 or 0-5% GVF, preferably 0% GVF, to the pump impellers. The multiphase inlet directs multiphase fluid, which is typically gas-rich, to the pump impellers. The power recovery turbine regains energy from the recycled liquid, reducing the power demand, and the injected liquid enhances pressure boosting of the gas of the multiphase fluid by rotation of liquid, which will be further explained below. The power recovery turbine is preferably of an inwards-flow type, such as a Francis type turbine, allowing liquid delivery close to the rotational axis of the pump, the benefit of which will be understood from the description below.

The pump and system of the invention use recycling of liquid in order to achieve pressure boosting of a gas-rich multiphase inlet flow. This is accomplished by mixing the gas rich multiphase flow with the liquid-rich liquid injection flow, increase the pressure with the pump impellers, separate the gas so the gas can be conveyed to the receiver and then bring a liquid-rich flow back to the low-pressure multiphase wellstream utilizing a power recovery turbine to regain some of the energy in the recycled flow.

With the pump and system of the invention, it is with a single turbomachine possible to effectively and more reliable pump or boost the pressure of fluid that can have a gas to liquid volume fraction from 0 to 100%, such as 0-95% GVF. A reduced power requirement is due to the energy effective solution, and an increased mean time between failures is due to the simplicity of the solution. A feasible fluid is wet gas with GVF in the range 70-100% but also pure liquid can be pumped. If the liquid fraction is high, the multiphase pump can pump liquid, if the liquid fraction is reduced toward dry gas, liquid is recirculated through the pump according to demand. The pump operates at lower rotational speed than a compressor, typically at about half to two thirds of the compressor speed (3-7000 rpm compared to 8-12000 rpm for gas compressors), nevertheless, gas compression is effective also for dry gas or near dry gas due to the liquid injection as prescribed. At the same time, contrary to a compressor, multiphase fluid of low GVF and pure liquid can easily be pumped effectively.

The pump and system is particularly feasible for use with small to medium sized wet gas field flows. Due to limitations with respect to power achievable with a single pressure boosting turbomachine, such as caused by mechanical instability or limits related to electric power, the largest gas field flow rates from large fields can be too large for a single machine for one field. However, also for large wet gas fields the system and pump of the invention are favourable since arranging multiphase pumps or systems of the invention in parallel or series still will reduce the number of machines and the complexity compared to state of the art technology.

With parallel systems or pumps of the invention, the flow or production assurance is increased compared to state of the art solutions with separate pump for liquid and compressor for gas. More specifically, production must typically be stopped with state of the art solutions if one of two turbomachines pump and compressor fail, but with parallel pumps or systems of the invention, production can continue even if one of the two turbomachines fail.

In preferable embodiments of the system and the pump of the invention, the multiphase inlet is arranged to the pump downstream of the turbine but upstream of the pump impellers, so as to mix lower pressure gas-rich inlet wellstream with equal pressure liquid-rich flow from the turbine, regaining energy of the recycled liquid rich flow in the turbine and compressing gas by rotating liquid in the impellers. Preferably, the coupling for liquid from the turbine is arranged so as to inject liquid into the pump impellers close to the rotational axis, preferably closer to the rotational axis than the gas rich inlet wellstream. Preferably recycled liquid is injected into the pump within two average fluid flow path diameters from the surface of the rotating hub or shaft, more preferably within one, and even more preferably within a half average fluid flow path diameter from the surface of the rotating hub or shaft, such as in an annular turbine liquid delivery flow cross section inside a coaxial outside annular multiphase fluid inlet arrangement, so liquid is injected in the volume where gas else could block the flow. In other words, a ring shaped liquid flow from the turbine is preferably arranged inside a ring shaped gas-rich flow from the multiphase inlet.

In a preferable embodiment of the system and pump, the separator is integrated with the pump and turbine into one machine, a cooler is preferably arranged in the pipe from the separator liquid outlet to the recovery turbine liquid inlet, and valves and instrumentation is preferably arranged in order to facilitate control. A pressure control valve can be arranged in the line to the turbine inlet, in order to ensure that recycled liquid delivery pressure from the turbine always can equal the multiphase inlet stream. In a preferable embodiment, a separate turbine-electric generator set is arranged in the turbine inlet pipe or a branch thereof in order to control pressure and regain energy, this can better ensure full regaining of energy and pressure control simultaneously in all operation modes.

The invention also provides a method of boosting the pressure of a multiphase fluid subsea by operating the system according to the invention, distinctive by recirculating liquid from the separator to the liquid inlet of the recovery turbine, at increasing rate at decreasing liquid contents of the multiphase fluid entering the multiphase pump, preferably so that the GVF ratio of fluid entering the pump impellers is 95% or lower, preferably 80% or lower, even more preferably 70% or lower. Preferably, the liquid injection or recirculation flow rate comprises 0-50%, more preferably 30-50% of the multiphase inlet flow rate.

The invention also allows boosting of dry gas by supplying a compatible liquid for the recycle boosting process. In one embodiment of the invention this is a method of boosting the pressure of a dry gas subsea by operating the pump according to the invention, distinctive by supplying a compatible liquid to the liquid inlet of the recovery turbine, at increasing rate at decreasing liquid contents of the multiphase fluid entering the multiphase pump, preferably so that the GVF ratio of fluid entering the pump impellers is 95% or lower, preferably 80% or lower, even more preferably 70% or lower, preferably the liquid is a hydrocarbon liquid that can be pumped further with the boosted gas, alternatively the liquid is hydrate inhibited produced water or seawater that can be separated downstream, used in another way or be pumped further with the gas.

The invention also provides use of a multiphase pump or a system according to the invention, for pressure boosting of fluid subsea. Also, the invention provides use of any feasible pump in combination with a turbine, for regaining energy from recirculated liquid or other higher pressure liquid flow, as described and illustrated in this document.

FIGURES

The invention is illustrated with six figures, namely:

FIG. 1 illustrating the principle of gas boosting by recycling liquid,

FIG. 2 illustrating a system and a multiphase pump of the invention, with combined pump, motor and power recovery turbine,

FIG. 3 illustrating a system of the invention with separate pump/motor and power recovery turbine/generator,

FIG. 4 illustrating the mixing section for liquid out of the recovery turbine and the gas-rich wellstream, in a system and pump of the invention,

FIG. 5 illustrating a combined pump init and separator, of a system and a pump of the invention, and

FIG. 6 illustrating details of the recovery turbine outlet and multiphase pump inlet, of a system and pump of the invention.

DETAILED DESCRIPTION

Reference is made to FIG. 1, illustrating the basic principle with a liquid recirculation flow which mixes with the gas-rich wellstream, the mixture is then boosted to a higher pressure in the multiphase pump section of the machine and then the gas is separated from the liquid in a separator and the liquid returns to the power recovery turbine where a considerable part of the liquid pressure energy is recovered to the shaft and is used to drive the multiphase pump. This invention allows liquid recirculation in order to avoid spending a lot of waste energy.

Reference is made to FIGS. 2 and 4, illustrating a system 1 and a multiphase boosting pump 2 of the invention. The multiphase pump 2 comprises a motor 3, multiphase impellers 4 arranged on the shaft, and a recovery turbine with runners 5 arranged on the same shaft 6 as the multiphase impellers.

The pump or machine is placed inside pressure housing 7, which has an inlet 8 for multiphase fluid, an outlet 9 from the pump and an inlet 10 for liquid injection or recirculation to the recovery turbine.

A separator 11 has an inlet 12 that is connected to the outlet 9 of the pump. A liquid outlet 14 from the separator is connected to the liquid inlet 10 on the machine so as to return liquid to the recovery turbine The separator comprises a multiphase outlet 13, said outlet exports mainly multiphase fluid gas if a liquid level in the separator is below the opening of the multiphase outlet pipe, if the liquid level is above the opening also liquid or mainly liquid is exported. The separator and outlet can have many embodiments. The pipe from the liquid outlet 14 from the separator comprises a cooler 15. A throttle valve 16 may be used to help control the operation. Valves or separate turbine-generator sets can be placed both on the inlet and outlet of the separator for this reason, and on the liquid inlet to the pump. Instead of or in addition to valves, the flow and pressure can be controlled by use of adjustable guide vanes upstream of the turbine runners as used for Francis type turbines and/or also adjustable guide vanes after the pump impellers.

The description outlines an arrangement with a separator outside the multiphase machine. The invention also allows for an arrangement where the multiphase machine is placed inside the separator, forming one compact unit, as illustrated in FIG. 5.

Alternatively, the multiphase pump and recovery turbine, and separator, are separate units, as illustrated in FIG. 3, illustrating a separate turbine-generator, where the turbine 5 and generator 20 is combined into one unit.

In operation a sensor (not illustrated) measuring the liquid contents, GVF ratio or equivalent is preferably monitoring the multiphase fluid inlet flow. A liquid level control mechanism, provided by instrumentation or by design as illustrated, ensures that sufficient liquid is retained in the separator to ensure sufficient liquid for recirculation of liquid through the multiphase pump for effective compression in situation with very dry multiphase inlet flow to the pump, i.e. at least 5% liquid flow for a dry gas inlet flow. The separator volume or separation effect should be sufficient for continuous recirculation of the necessary liquid recirculation flow rate from the separator in order to manage the desired boosting. In situations with very dry multiphase fluid, the liquid recirculation must be sufficient to ensure effective pressure boosting, a GVF of 95% or lower through the impellers is considered feasible for effective pressure boosting.

Reference is made to FIG. 6, illustrating a preferred embodiment of mixing liquid and gas in the turbine-multiphase inlet-impeller coupling. Liquid injection from the recovery turbine 17 is made close to the shaft and the gas-rich wellstream inlet 18 is made on the outer periphery. It is considered a big advantage to inject liquid close to the rotational axis of the pump shaft, because this is where gas accumulates according to tests and simulations, whilst pressure build up in substance takes place further out on the impeller blades. The injected liquid will thereby entrain the gas to the outer parts of the impeller blades for more effective pressure boosting. As mentioned, liquid should be injected into the pump within two average fluid flow path diameters from the surface of the rotating hub or shaft, more preferably within one, and even more preferably within a half average fluid flow path diameter from the surface of the rotating hub or shaft, such as in an annular turbine liquid delivery flow cross section inside a coaxial outside annular multiphase fluid inlet arrangement, so liquid is injected in the volume where gas else could block the flow. In other words, a ring shaped liquid flow from the turbine is preferably arranged inside a ring shaped gas-rich flow from the multiphase inlet. Average fluid flow path diameter is the average of two orthogonal diameters of the flow path between or along impeller blades, at the inner of the flow path, at the shaft or hub.

The most preferred embodiment is illustrated and described in detail, for the system and the pump, respectively, however, many equivalent embodiments are possible. The pump must not be a centrifugal pump, also other rotordynamic pump types as mixed flow, axial or helico-axial pumps and also other pump types such as a displacement type pump is feasible, such as a piston, plunger, screw or gear pump. Further, the separator must not be a gravity type separator, it can be any type feasible for sufficient separation in order to recirculate more or less pure liquid, such as 0-5% GVF liquid, to the turbine, for example cyclone separators or other separators using rotation. Further, the turbine can be any turbine feasible for regaining energy from the liquid, also displacement type turbines like piston, plunger, screw or other rotordynamic radial, mixed flow or axial types such as Francis, Kaplan or propeller type turbines. It is possible to combine rotordynamic and displacement machines with respect to turbine and pump, however, separate shafts, an additional generator on the turbine shaft or gears or couplings can be required to couple different turbine and pump designs, or arranging a generator to the turbine and arranging an electric power supply from the generator to the pump motor. Furthermore, the pump motor can be hydraulic, allowing easy regaining of energy from the recirculated liquid.

The system of the invention may include any feature as herein described or illustrated, in any operative combination, each such operative combination is an embodiment of the invention. The multiphase pump of the invention may include any feature as herein described or illustrated, in any operative combination, each such operative combination is an embodiment of the invention. The method and the use of the invention may include any step or feature as herein described or illustrated, in any operative combination, each such operative combination is an embodiment of the invention. 

1. A system for multiphase pumping, the system comprising a multiphase pressure boosting pump and a separator, a multiphase fluid inlet connected to the pump, a multiphase outlet from the separator, an outlet of the pump arranged to an inlet of the separator, a liquid outlet of the separator arranged to a liquid inlet on the pump, and a power recovery turbine arranged between said liquid outlet of the separator and said liquid inlet on the pump, where the pressure at the outlet of the turbine is equal to or substantially equal to the pressure at the inlet on the pump.
 2. The system according to claim 1, wherein the system is adapted for application subsea.
 3. The system according to claim 1, wherein the multiphase inlet is arranged to the pump downstream of the turbine but upstream of the pump impellers, so as to mix lower pressure gas-rich inlet wellstream with equal pressure liquid-rich flow from the turbine, regaining energy of the recycled liquid rich flow in the turbine and compressing gas by rotating liquid in the impellers.
 4. The system according to claim 1, wherein the coupling for liquid from the turbine is arranged so as to inject liquid into the pump impellers close to the rotational axis, preferably within two average fluid flow path diameters from the surface of the rotating hub or shaft, more preferably within one, and even more preferably within a half average fluid flow path diameter from the surface of the rotating hub or shaft, such as in an annular turbine liquid delivery flow cross section inside an annular multiphase fluid inlet arrangement.
 5. The system according to claim 1, wherein a cooler is arranged in the pipe from the separator liquid outlet to the pump liquid injection inlet, the impeller blades are multiphase fluid impeller blades, the separator is preferably integrated with the pump and turbine into one machine.
 6. A multiphase pressure boosting pump comprising a motor, one or more impellers arranged on a shaft common with a motor shaft or coupled to the motor, a pressure housing, a multiphase fluid inlet and an outlet, the pump further comprises a liquid inlet and a power recovery turbine, with the power recovery turbine arranged between the liquid inlet and the pump impellers.
 7. The multiphase pressure boosting pump according to claim 6, wherein the pump is adapted for application subsea.
 8. The multiphase pressure boosting pump according to claim 6, wherein the liquid injection inlet is arranged into an upstream side of the power recovery turbine so as to regain energy in the turbine and enhance gas compression by rotation of injected liquid by the impellers.
 9. The multiphase pressure boosting pump according to claim 6, wherein the coupling for liquid from the turbine is arranged so as to inject liquid into the pump impellers close to the rotational axis, preferably within two average fluid flow path diameters from the surface of the rotating hub or shaft, more preferably within one, even more preferably within a half average fluid flow path diameter from the surface of the rotating hub or shaft, such as in an annular turbine liquid delivery flow cross section inside an annular multiphase fluid inlet arrangement.
 10. The multiphase pressure boosting pump according to claim 6, wherein the impellers comprise multiphase fluid blades, and the injected liquid has a velocity component parallel to the multiphase velocity direction at the coupling from turbine to impellers, preferably coaxial around a rotating shaft common for the motor, impellers and turbine, but coaxial inside multiphase fluid.
 11. A method of boosting the pressure of a multiphase fluid, preferably subsea, by operating the system according to of claim 1, wherein liquid recirculates from the separator to the liquid inlet of the recovery turbine, at increasing rate at decreasing liquid contents of the multiphase fluid entering the multiphase pump.
 12. The method according to claim 11, wherein the liquid injection or recirculation flow rate comprises 30-50% of the multiphase inlet flow rate.
 13. A method of boosting the pressure of a dry gas, preferably subsea, by operating the pump according to of claim 6, comprising supplying a compatible liquid to the liquid inlet of the recovery turbine, at increasing rate at decreasing liquid contents of the multiphase fluid entering the multiphase pump.
 14. (canceled)
 15. (canceled)
 16. The method according to claim 11, wherein the GVF ratio of fluid entering the pump impellers is 95% or lower.
 17. The method according to claim 16, wherein the GVF ratio of fluid entering the pump impellers is 80% or lower.
 18. The method according to claim 17, wherein the GVF ratio of fluid entering the pump impellers is 70% or lower.
 19. The method according to claim 13, wherein the GVF ratio of fluid entering the pump impellers is 95% or lower.
 20. The method according to claim 19, wherein the GVF ratio of fluid entering the pump impellers is 80% or lower.
 21. The method according to claim 20, wherein the GVF ratio of fluid entering the pump impellers is 70% or lower.
 22. The method according to claim 13, wherein the liquid is a hydrocarbon liquid that can be pumped further with the boosted gas.
 23. The method according to claim 13, wherein the liquid is hydrate inhibited water. 